Triggered arc discharge device having a liquid cathode



y 2, 11967 R. E. BRITTAIN ETAL TRIGGERED ARC DISCHARGE DEVICE HAVING ALIQUID CATHODE Filed May 18, 1964 1 4 2 l 1| I i /n N a o v m 5 g 5 ML IINVENTO'RS QTTOENEYQ I United States Patent 3,317,776 TRIGGERED ARCDISCHARGE DEVICE HAVING A LIQUID CATHODE Raymond Ernest Brittain,Sutton, and William Robert Mayberry, Hounslow, England, assignors to TheM-O Valve Company Limited, London, England Filed May 18, 1964, Ser. No.368,201 Claims priority, application Great Britain, May 20, 1963,19,949/63 7 Claims. (Cl. 313-170) This invention relates to electricdischarge devices.

The invention is concerned in particular with electric discharge devicesof the kind having a sealed envelope within which are disposed a cathodein the form of a pool of conducting liquid, an anode and a triggerelectrode interposed between the anode and cathode, the envelope havinga low pressure gas filling and the arrangement being such that, with theanode maintained positive with respect to the cathode by a voltage lyingWithin a range below the anode breakdown voltage, a discharge betweenthe anode and cathode may be initiated by applying a suitable triggeringvoltage between the trigger electrode and the cathode. By the anodebreakdown voltage is meant the lowest value of voltage applied betweenthe anode and cathode, in a sense which makes the anode more positive,for which a discharge would occur with the trigger electrode maintainedat the same potential as the cathode; by a low pressure gas filling ismeant a gas filling at a pressure less than that pressure of gas fillingfor which the anode breakdown voltage would have its minimum value inrespect of devices of identical construction and having gas fillings ofthe same composition.

One possible application of an electric discharge device of the kindspecified is as a surge diverter. For such an application the device maybe required to have a relatively high anode breakdown voltage (say of atleast five kilovolts), while at the same time it should be possiblereliably to initiate a discharge between the anode and cathode using arelatively small trigger power when the anode-cathode voltage isconsiderably less than the anode breakdown voltage. The device should becapable of passing relatively high peak anode currents, but Will notnormally be required to have nearly such a high ratin in respect of meananode cur-rent, since anode current will normally only be passed forshort periods which are not rapidly repeated.

In practice it has been found some-what diflicult to meet theserequirements in a device which is relatively inexpensive to manufacture,and it is therefore an object of the invention to provide an electricdischarge device of the kind specified which is improved in thisrespect.

According to one aspect of the invention, in an electric dischargedevice of the kind specified the anode is effectively in the form of anend portion of a metal rod which is disposed coaxially within a tubularinsulator, the insulator being spaced from the rod over a length of therod contiguous With said end portion; the trigger electrode iseffectively in the form of a tubular metal member which is closed at oneend by a metal closure member, the tubular metal member being supportedat its open end from the insulator so as to be disposed coaxial with therod and with its closed end spaced from said end portion of the rod, theclosure member being apertured to permit the passage of a dischargebetween the anode and cathode, the spacing between the closure memherand the adjacent end of the rod lying in the range O.52 millimetres andthe spacing between the rod and the tubular metal member lying in therange 1-4 millimetres; and the pressure of the gas filling has a valuewithin the range 0.3-1.0 times P, where P is that prespensivetechniques.

3,317,776 Patented May 2, 1967 sure of gas filling for which the valueof the anode breakdown voltage would be five kilovolts in a device ofidentical construction and having a low presure gas filling of the samecomposition.

It may be noted that in discharge devices of the kind specified, for agiven construction and composition of the gas filling the value of theanode breakdown voltage decreases with an increase of the pressure ofthe gas filling, while at the same time it becomes easier to meet therequirement concerning reliable triggering of the device. Moreover, fora given composition and pressure of the gas filling, the value of theanode breakdown voltage is largely dependent on the length of theelectric lines of force between the anode and the trigger electrode, thebreakdown voltage increasing with a decrease of this length at least tothe point at which it becomes so small that breakdown is possible byVirtue of effects associated with the occurrence of a high value of theelectric field strength.

Thus the use of relatively small values for the spacings between theanode and the trigger electrode, :as specified above, makes it possible,with a given composition of the gas filling, to obtain a relatively highvalue for P, and hence to achieve a satisfactory compromise between therequirements concerning the anode breakdown voltage and the ease ofreliable triggering over a wider range of pressures of the gas fillingthan would be possible when using larger values for these spacings. Themanufacture of the device is thus not rendered diflicult and expensiveby virtue of a requirement to control the pressure of the gas fillingwithin very narrow limits. It will be appreciated that with such anarrangement it is necessary to control the values of thespacings betweenthe anode and the trigger electrode relatively closely duringmanufacture; the simple and rigid form specified above for theanode-trigger electrode assembly, which is in itself of a relativelyinexpensive nature, enables this requirement to be met without involvingthe use of complicated and ex- Further, the form of this assembly issuch that it is readily able to withstand the mechanical stresses whichmay be associated with the passage of relatively high peak anodecurrents.

In an electric discharge device in accordance with the invention thecathode suitable consists of mercury containing a small proportion of atleast one of the alkaline earth metals barium, strontium, calcium andmagnesium, the total amount of the additive being such that if themercury is rolled over a glass surface it tends to stick to thatsurface. Preferably the additive consists solely of barium, in whichcase the amount incorporated in the mercury may suitably lie in therange 1-10 milligrams per cc. of mercury.

The use of an alkaline earth metal additive in such an amount results inan improvement in the triggering characteristics of the device ascompared with a case in which a plain mercury cathode is used. Inparticular, for a given device and a given trigger power, it makes itpossible to obtain reliable triggering down to lower values of appliedanode-cathode voltage. Alternatively, it makes it possible, with adevice of given construction and composition of the gas filling, toobtain reliable triggering with a given trigger power and appliedanode-cathode voltage while using a lower pressure for the gas filling.This feature may thus contribute further to the possibility of obtaininga satisfactory compromise between the requirements concerning the anodebreakdown voltage and the ease of reliable triggering over a relativelyWide range of pressures of the gas filling.

One electric discharge device will now be described by Way of examplewith reference to the accompanying drawing, which is a sectional view ofthe device.

The device has a sealed glass envelope 1 within which are disposed acathode 2, and anode 3 and a trigger electrode 4. The envelope 1 isgenerally of circular cylindrical form, and in operation of the deviceis disposed with its longitudinal axis vertical, the cathode 2, which isin the form of a pool of mercury having a volume of about two cc., beingdisposed at the lower end of the envelope 1 in direct contact with theinside of the envelope 1. At this end, the envelope 1 is narrowed downto form a hollow projecting pip 5, the mercury filling the space withinthe pip 5 and extending into the space above it; a connector for thecathode 2 is provided in the form of a metal rod 6 which is sealedthrough the end of the pip 5 so that one end of the rod 6 is immersed inthe mercury.

At its other end the envelope 1 is formed with a reentrant portion 7,through which is sealed an assembly including a glass tube 8 and atungsten rod 9 which are disposed coaxial with each other, the rod 9having a circular cross-section of diameter four millimetres and thetube 8 having a circular cross-section with internal and externaldiameters respectively of 6.5 millimetres and one centimetre. The outerend of the tube 8 is sealed on to the rod 9 at a point near the outerend of the rod 9, while the tube 8 is sealed through the portion 7 ofthe envelope 1 at a point about midway along its length. The inner endof the tube 8 projects for a distance of one millimetre beyond the innerend of the rod 9, the portion of the rod 9 adjacent this end effectivelyconstituting the anode 3 of the device. It will be appreciated that theremainder of the rod 9, which has a total length of about eightcentimetres, constitutes a connector for the anode 3.

The trigger electrode 4 includes a nickel tube 10 having a length of 1.5centimetres and an internal diameter of one centimetre, the tube beingprovided at one end with an outwardly projecting radial flange 11 andbeing disposed so that it fits around the tube 8 with its flanged end inregister with the inner end of the tube 8; the tube 10 may for examplebe formed by bending round a flat sheet of nickel and clamping the freeedges together. The flanged end of the tube 10 is closed by a closuremember in the form of an iron disc 12 having a diameter substantiallyequal to the outer diameter of the flange 11, the disc 12 being weldedat its periphery to the flange 11 and having one main face abuttingagainst the end of the tube 8 so that the trigger electrode 4 isaccurately located in a longitudinal direction. The disc 12 has formedat its centre a circular aperture 13 of diameter one millimetre, and hasprojecting from its periphery a bent tab 14 whose purpose is explainedbelow. It will be appreciated from the dimensions given above that thespacing between the disc 12 and the adjacent end of the rod 9 is onemillimetre and the spacing between the rod 9 and the tube 10 is threemillimetres. It will be further appreciated that the spacing of the tube8 from the rod 9 over a length of the rod 9 contiguous with the anode 3reduces the risk of breakdown occurring between the anode 3 and thetrigger electrode 4 through or along the surface of the tube 8. Thetotal length of the envelope 1 is such that the spacing between thecathode 2 and the trigger electrode 4, which is not critical, has avalue of a few centimetres, and a connector for the trigger electrode 4is provided in the form of a metal rod 15 sealed through the side wallof the envelope 1 at a position which is more dis- :tant from thecathode 2 than is the trigger electrode 4, the trigger electrode 4 beingconnected to the rod 15 by a suitable lead 16.

The envelope 1 is filled with argon at a nominal pressure of 0.25 torr,the tolerance allowed on this parameter during manufacture being about'10%. The value of the anode breakdown voltage for a device having theconstruction described above and with an argon filling at a pressure of0.25 torr is about six kilovolts; by way of comparison it may be statedthat the pressure of gas filling for which the anode breakdown voltagewould have its minimum value in respect of devices of identicalconstruction and having argon fillings is several times greater, andthat the value of the pressure P in respect of such a device is about0.35 torr.

The device is provided with a number of barium getter elements 17 of thetype conventionally used in vacuum tubes, these elements being supportedfrom the tab 14 So as to be disposed to one side of the triggerelectrode 4 and the total amount of barium incorporated in them beingabout five milligrams. After the envelope 1 has been filled with gas thegetters 17 are fired by eddy current heating to evaporate the barium,and the major part of it becomes incorporated in the mercury of thecathode 2. As a result, the state of the mercury is altered in such amanner that, if it is caused to roll over the inside of the envelope 1by tilting the device, it exhibits a tendency to stick to the inside ofthe envelope 1. The improved triggering characteristics thereby obtainedmay be illustrated by considering a case in which the device istriggered by means of a pulse corresponding to one half cycle of asinusoidal alternating voltage of frequency 50 kc./s. and amplitude twokilovolts, the pulse being applied between the trigger electrode 4 andthe cathode 2 so as to drive the trigger electrode 4 more positive andthe current flowing through the trigger electrode 4 being limited to amaximum value of 10 milliamperes. With such an arrangement it is foundthat a discharge between anode 3 and cathode 2 may be reliably initiatedin the device described above with values of the applied anodecathodevoltage down to less than one kilovolt, whereas in an otherwise similardevice having a plain mercury cathode the corresponding figure would befour kilovolts.

In alternative arrangements, a similar improvement in triggeringcharacteristics may be brought about if the barium incorporated in themercury is replaced by an equivalent quantity of strontium, calcium ormagnesium.

Finally, it may be noted that the device described above is capable ofpassing peak anode currents having a value of 2000 amperes.

We claim:

1. An electric discharge device having a sealed envelope within whichare disposed: a cathode in the form of a pool of conducting liquid; ananode which is eifectively in the form of an end portion of a metal rodwhich is disposed coaxially within a tubular insulator, the insulatorbeing spaced from the rod over a length of the rod contiguous with saidend portion; and a trigger electrode interposed between the anode andcathode, the trigger electrode being efiectively in the form of atubular metal member which is closed at one end by a metal closuremember, the tubular metal member being supported at its open end fromthe insulator so as to be disposed coaxial with the rod and with itsclosed end spaced from said end portion of the rod, the closure memberbeing apertured to permit the passage of a discharge between the anodeand cathode, the spacing between the closure member and the adjacent endof the rod lying in the range 0.5-2 millimetres and the spacing betweenthe rod and the tubular metal member lying in the range 1-4 millimetres;the envelope having a low pressure gas filling whose pressure has avalue within the range 0.31.0 times P, where P is that pressure of gasfilling for which the value of the anode breakdown voltage would be fivekilovolts in a device of identical construction and having a lowpressure gas filling of the same composition.

2. An electric discharge device according to claim 1, in which thetubular metal member fits closely around the tubular insulator.

3. An electric discharge device according to claim 1, in which thetubular insulator is sealed through the envelope at a point intermediatealong the length of the insulator, the tubular metal member beingsupported from the inwardly projecting end of the insulator, and theoutwardly projecting end of the insulator being sealed on to the metalrod at a point intermediate along the length of the rod.

4. An electric discharge device according to claim 1, in which thetubular insulator is of glass.

5. An electric discharge device according to claim 1, in which the gasfilling is of argon at a pressure in the range 0.2-0.3 torr.

6. An electric discharge device having a sealed envelope within whichare disposed: a cathode in the form of a pool of conducting liquid; ananode which is effectively in the form of an end portion of a metal rodwhich is disposed coaxially within a tubular insulator, the insulatorbeing spaced from the rod over a length of the rod contiguous with saidend portion; and a trigger electrode interposed between the anode andcathode, the trigger electrode being efiectively in the form of atubular metal member which is closed at one end by a metal closuremember, the tubular metal member being supported at its open end fromthe insulator so as to be disposed coaxial with the rod and with itsclosed end spaced from said end portion of the rod, the closure memberbeing apertured to permit the passage of a discharge between the anodeand cathode, the spacing between the closure member and the adjacent endof the rod lying in the range 0.5-2 millimetres and the spacing betweenthe rod and the tubular metal member lying in the range 1-4 millimetres;the envelope having a low pressure gas filling whose pressure has avalue within the range 0.3- 1.0 times P, where P is that pressure of gasfilling for which the value of the anode breakdown voltage would be fivekilovolts in a device of identical construction and having a lowpressure gas filling of the same composition, and the cathode consistingof mercury containing a small proportion of an additive selected fromthe group consisting of barium, strontium, calcium, magnesium, andmixtures of these metals, the total amount of the additive being suchthat if the mercury is rolled over a glass surface it tends to stick tothat surface.

7. An electric discharge device having a sealed envelope filled withargon at a pressure in the range 0.2-0.3 torr, the envelope havingdisposed within it: a cathode in the form of a pool of mercurycontaining barium in an amount lying in the range 1-10 milligrams percc. of mercury; an anode Which is efiectively in the form of an endportion of a metal rod which is disposed coaxially within a tubularglass insulator, the insulator being spaced from the rod over a lengthof the rod contiguous with said end portion and being sealed through theenvelope at a point intermediate along the length of the insulator, andthe outwardly projecting end of the insulator being sealed on to the rodat a point intermediate along the length of the rod; and a triggerelectrode interposed between the anode and cathode, the triggerelectrode being effectively in the form of a tubular metal member whichis closed at one end by a metal closure member, the tubular metal memberfitting closely around the inwardly projecting end of the insulator soas to be supported therefrom with the closure member spaced from saidend portion of the rod, the closure member being apertured to permit thepassage of a discharge between the anode and cathode, the spacingbetween the closure member and the adjacent end of the rod lying in therange 0.5-2 millimetres and the spacing between the rod and the tubularmetal member lying in the range 1-4 millimetres.

References Cited by the Examiner UNITED STATES PATENTS 2,675,495 4/1954DeLany 313328 X 2,976,451 3/1961 Reiling 313-328 X 2,985,786 5/1961Humphrey 313-225 X JAMES W. LAWRENCE, Primary Examiner. S. D. SCHLOSSER,Assistant Examiner.

1. AN ELECTRIC DISCHARGE DEVICE HAVING A SEALED ENVELOPE WITHIN WHICHARE DISPOSED: A CATHODE IN THE FORM OF A POOL OF CONDUCTING LIQUID; ANANODE WHICH IS EFFECTIVELY IN THE FORM OF AN END PORTION OF A METAL RODWHICH IS DISPOSED COAXIALLY WITHIN A TUBULAR INSULATOR, THE INSULATORBEING SPACED FROM THE ROD OVER A LENGTH OF THE ROD CONTIGUOUS WITH SAIDEND PORTION; AND A TRIGGER ELECTRODE INTERPOSED BETWEEN THE ANODE ANDCATHODE, THE TRIGGER ELECTRODE BEING EFFECTIVELY IN THE FORM OF ATUBULAR METAL MEMBER WHICH IS CLOSED AT ONE END BY A METAL CLOSUREMEMBER, THE TUBULAR METAL MEMBER BEING SUPPORTED AT ITS OPEN END FROMTHE INSULATOR SO AS TO BE DISPOSED COAXIAL WITH THE ROD AND WITH ITSCLOSED END SPACED FROM SAID END PORTION OF THE ROD, THE CLOSURE MEMBERBEING APERTURED TO PERMIT THE PASSAGE OF A DISCHARGE BETWEEN THE ANODEAND CATHODE; THE SPACING BETWEEN THE CLOSURE MEMBER AND THE ADJACENT ENDOF THE ROD LYING IN THE RANGE 0.5-2 MILLIMETRES AND THE SPACING BETWEENTHE ROD AND THE TUBULAR METAL MEMBER LYING IN THE RANGE 1-4 MILLIMETRES;THE ENVELOPE HAVING A LOW PRESSURE GAS FILLING WHOSE PRESSURE HAS AVALUE WITHIN THE RANGE 0.3-1.0 TIMES P, WHERE P IS THAT PRESSURE OF GASFILLING FOR WHICH THE VALUE OF THE ANODE BREAKDOWN VOLTAGE WOULD BE FIVEKILOVOLTS IN A DEVICE OF IDENTICAL CONSTRUCTION AND HAVING A LOWPRESSURE GAS FILLING OF THE SAME COMPOSITION.